Field of the Invention
This invention relates to austenitic alloys, and in particular, relates to austenitic alloys of the kind which find the application in the chemical process industries, the petrochemical industry, the pulp and paper industry, the power-plant scrubber market, and any other markets requiring a material with high degree of corrosion resistance to chloride pitting, as well as general corrosion resistance.
Those skilled in the art are familiar with various steels and alloys that have hitherto been known for the purposes indicated above. These include the austenitic alloys of (1) the Deverell U.S. Pat. Nos. 4,007,038 and 4,043,838; (2) the alloy of the McCunn et al. U.S. Pat. No. 4,545,826; and (3) the alloy "Avesta 254 SMO". These known materials have in common that they typically contain about 20% of chromium, a large proportion of nickel (24% in the case of the alloys (1) and (2) and 18.0% in the case of alloy (3)), about 6.5% of molybdenum, and the balance iron. The alloy (2) also contains, according to the McCunn et al. patent, 0.15 to 0.30% of nitrogen, though the usual commercial material of alloy (2) is made with a nitrogen aim content on the order of 0.18 to 0.25%. The alloy (3) typically contains 0.2% of nitrogen and 0.7% of copper. The balance is substantially iron, except for incidental impurities in each alloy.
Alloys of the above-indicated compositions are known to exhibit a desirable combination of hot-workability, strength, and resistance to corrosion in various media.
It is known that the alloy material having a chemical composition generally similar to the three above-mentioned alloys usually have substantially austenitic microstructure at room temperature, but there is a tendency, with these highly alloyed materials, to have some development of other microstructural phases, such as the sigma phase and the chi phase. In general the development of these other phases, the sigma phase and the chi phase, is to be avoided, because of the unfavorable effects on the hot workability, the strength, or certain of the other properties of the alloys involved. While the use of alloy materials less highly alloyed would be desirable from the standpoint of avoiding the development of sigma or chi phases, such materials are often accompanied by a decrease in strength and/or corrosion resistance.
In the testing and development of alloys of this kind, it is common, in addition to determining the usual mechanical properties (ultimate tensile strength, yield strength, and elongation), to conduct tests in accordance with ASTM Standard G-48 to determine the CCCT (i.e., the Critical Crevice Corrosion Temperature). The above-mentioned McCunn et al. Patent No. 4,545,826 discloses how the CCCT is favorably influenced by the use of a greater nitrogen content in the alloy. One example at 0.25 weight percent nitrogen achieved, when heat treated and water quenched in a specified manner, a CCCT of about 108 degrees Fahrenheit (42 degrees C.).
In another kind of known corrosion test which is applied to alloys of this sort, the resistance of the materials to stress-corrosion cracking is determined by subjecting samples of the material to exposure in a boiling salt solution. In one of the severest tests of this kind, there is used a boiling aqueous solution containing 42 weight percent of magnesium chloride, a medium in which samples of the alloy (1) survive about 96 hours and samples of the alloy (2) survive 500 hours or more. Other austenitic materials, such as AISI Type 316L stainless steel, in such a test, fail in a time of the order of 20 hours.
It is known for high Cr and Ni bearing materials to maintain the sulfur content of the alloy at a low level, up to 0.006 weight percent, as a measure intended to preserve or promote the hot workability of the maaterial. Alternatively, it may be desirable or necessary to treat the steel with calcium and/or cerium for this purpose as disclosed in the Deverell patents 4,007,038 and 4,043,838.
It is known, in accordance with the prior art, that the presence of molybdenum in greater amounts promotes the obtaining of a higher CCCT, but at the same time, it has a detrimental effect on the phase stability and hot workability of the alloy.
What is needed is a Ni-Cr-Mo containing austenitic stainless steel which remains adequately hot-workable and avoids the development of unwanted sigma-phase microstructure, affording desirably high CCCT values on the order of 49 degrees Centigrade (120 degrees F.) or higher, adequate strength and ductility, and a desirable level of resistance to stress corrosion cracking without the high cost of currently known nickel-base alloy products.